The applicant's U.S. Pat. No. 4,796,009, issued Jan. 3, 1989, discloses one type of high-powered electronic warning apparatus to which the present invention is applicable; and another is disclosed in U.S. Pat. No. 4,344,504. In these and similar electronic devices, electrical energy is converted to accoustical energy by means of a number of identical electromagnetic drivers that cooperate with a horn or horns through which the sound energy produced by the drivers is beamed or distributed.
As compared with the earlier-developed electromechanical sirens, an electronic warning device has the advantage of being able to produce highly amplified voice outputs as well as tone outputs like those of an electromechanical siren. A more important consideration, however, is that an electronic device is capable of emitting an acoustic output that is horizontally beamed, to be audible at great distances from the apparatus, whereas a electromechanical siren inherently produces an output that disperses vertically. Thus, for warning apparatus of more than about 5 kW power input, the cost of an electronic tone-generating device is lower than that of an equally effective electromechanical device, and this cost differential increases with increasing input power. For this reason, electronic warning devices are now generally preferred over electromechanical devices where high-volume tone outputs are needed, even in the many cases where the voice capabilities of the electronic devices are not utilized.
Each of the drivers of an electronic warning device has a rigid housing comprising dome shaped front and rear castings that are coaxially secured to one another in clamping relationship to the rim of a diaphragm. The diaphragm divides the housing into a totally enclosed rear chamber wherein there is a powerful permanent magnet and a front chamber which serves as an acoustical impedance chamber and in which there is a driver port that is opposite the diaphragm and coaxial with it.
In each driver a coil is secured to the diaphragm, at its rear side. When the coil is energized with alternating current, it cooperates with the magnet to impart back and forth oscillations to the diaphragm that are in step with the alternations of the applied current.
The front chamber of the driver housing, which has a restricted outlet defined by the relatively small driver port, acoustically loads the diaphragm to limit the amplitude of its oscillatory excursions and thus prevents it from being overstressed. The driver port ordinarily opens into a further acoustical impedance chamber, which may comprise the narrow throat of an exponential horn. In the case of the apparatus of U.S. Pat. No. 4,796,009, the further impedance chamber is drum-like, the drivers are mounted around its periphery, and an exponential horn extends coaxially from one of its end walls or each of them. In the apparatus of U.S. Pat. No. 4,344,504 there is a horn for each driver and the several horns open into a common waveguide.
Heretofore the source of energizing current for such an electronic alerting and warning device has comprised a set of heavy-duty storage batteries that were connected through a suitable switch with an amplifier system. For siren-like tone emission the amplifier system was modulated by means of a tone generator of the desired frequency or frequencies. With the provision of suitable means comprising a microphone and a selector switch, the amplifier system could be voice modulated for delivery of spoken messages.
In the course of time there has been a virtual standardization of the drivers used with commercial electronic warning apparatus and of the amplifier systems used with those drivers. The typical driver is sold by Atlas Sound Company as its Model SA 370. It is duty rated at 100 W at 40 V and has an a.c. resistance of 11 ohms. The typical amplifier is made up of a number of modular amplifier units, each having a 200 W output to be capable of powering a pair of drivers that are connected to its output in parallel. As installed in prior apparatus, the several amplifier modules have had their power and modulation inputs connected in parallel, but each module has had its output connected only with an associated pair of drivers, so that a relatively large number of wires had to extend between the amplifier system, which is usually at or near ground level, and the drivers, which are usually mounted on a high mast or other elevated support. Such modular amplifier systems have come into widespread commercial use in electronic warning devices because they are readily available, are known to be reliable, and are versatile because they can be readily assembled into warning devices comprising any desired even number of drivers.
The present invention resides in substantial part in the recognition of serious disadvantages inherent in the heretofore conventional means for energizing the drivers to produce tone signals, comprising storage batteries, a tone generator and amplifier modules. The underlying factor is that the conventional amplifier module cannot be modulated for a full power sine-wave output, since this would require its transistors to provide a substantial amount of resistance during a major portion of each sine-wave cycle, causing current to be converted to heat energy at such a rate as to overheat the module and destroy it. The tone generator therefore imposes a square-wave modulation on the amplifier modules, each of which accordingly delivers a square-wave output to its associated drivers. Since the square-wave amplifier output is always either all current and no voltage or no current and all voltage, there is no substantial heating of the amplifier modules.
However, the application of this square-wave a.c. energization to the drivers has been the cause of heretofore unrecognized problems and inefficiencies. When a driver is energized with an a.c. of square-wave form, each cycle of that current tends to propel the diaphragm of the driver first in one direction at maximum force and then in the opposite direction at maximum force, with an instantaneous force reversal at each change in phase. In effect, the square-wave current seeks to impose upon the diaphragm an infinite acceleration first in one direction and then in the other, jerking it abruptly back and forth, in contrast to the smooth and gradual accelerations that a sine-wave current tends to produce.
The high and abrupt accelerations of the diaphragm that result from square-wave current energization are obviously stressful and have the effect of shortening its useful life. In fact, failure of drivers is not uncommon, and each of the commercial amplifier modules has a signal light that warns when one of its associated drivers has failed.
Another and very important disadvantage of energizing a driver with a square-wave a.c. is that the driver produces a relatively poor acoustic output. For one thing, the power applied to the driver must be substantially lower than would be feasible with sine-wave energization, to avoid overheating of the driver coil as well as mechanical overstressing of the diaphragm. The driver coil has a high impedance to a square-wave alternating current, inasmuch as a square wave can be regarded as a sine wave of the same frequency that has all of its harmonics added to it, to thus constitute the equivalent of a very high frequency, and the impedance of the driver coil is linearly proportional to the frequency of the a.c. applied to it. In addition, the inherently inefficient abrupt acceleration forces that square-wave energization imposes upon the diaphragm reduce the acoustic output of the driver to a value substantially below what it would be if the same input power were applied to the driver in sine-wave form.
Apparently these inefficiencies of the prior energizing means have not heretofore been understood or appreciated, probably because there was no evident need for thinking about them. Any desired acoustical output could be obtained by simply using as many drivers as were needed for that output, using the number of amplifier modules appropriate for that number of drivers, and using enough batteries to power the apparatus.
But a high-powered warning device is expensive when it is designed to produce a given acoustical output while accommodating the unrecognized inefficiencies. Obviously it is more costly by reason of its having more drivers, amplifier modules and batteries than are truly needed. In apparatus having a horn for every driver, as in the widely used arrangement disclosed in U.S. Pat. No. 4,344,504, there is the further very substantial cost of an increased number of horns to accommodate the increased number of drivers. In turn, each additional horn increases the bulk of the acoustic apparatus, subjecting it to increased wind loads as well as increasing its weight, thus requiring that it be mounted on a sturdier and more expensive mast.
The market for public alerting and warning devices is a highly competitive one. Most installations of such apparatus are contracted for on the basis of bids. The advertisements for such bids usually lay down rigid performance specifications, and therefore price is the principal criterion upon which contracts are awarded. The competitive pressure for lower prices is in itself evidence of the unboviousness of the present invention, for if there had been any obvious method or means for achieving a substantial reduction in cost of such devices commercial necessity would surely have compelled skill in the art to seize upon that expedient.